In recent years, in liquid crystal displays, organic and inorganic electroluminescence devices, electronic papers, etc., a film or a glass substrate having a transparent conductive layer has been used as an electrode on the light-emitting side (see, for example, Japanese Laid-Open Patent Publication Nos. 08-180974, 09-147639, 10-162961, and 11-224782).
The transparent conductive layer is generally composed of an indium tin oxide, a zinc oxide, a tin oxide, etc., and has to be thick and uniform to achieve low resistance. Thus, the layer is disadvantageous in low light transmittance, high cost, and that a high temperature treatment is needed in the formation process. Particularly in the case of forming the transparent conductive layer on the film, the resistance can be lowered only to a limited extent.
In view of improving the problem, a method containing adding a conductive component such as a metal wire to the transparent electrode layer (Japanese Laid-Open Patent Publication No. 09-147639), a method containing forming a conductive metal busline on the transparent electrode layer (a transparent positive electrode substrate) (Japanese Laid-Open Patent Publication Nos. 08-180974 and 10-162961), and a method containing forming a network-patterned metal wire structure on the transparent electrode layer (an upper electrode) (Japanese Laid-Open Patent. Publication No. 2005-302508) have been proposed.
Meanwhile, a car light has an illuminance reduction problem. The illuminance of the car light may be reduced due to the following causes:    (1) adhesion and accumulation of snow on the outer circumferential surface of the front cover,    (2) adhesion and freezing of water such as rain water or car wash water on the outer circumferential surface of the front cover, and    (3) progression of (1) and (2) due to use of an HID lamp light source having a high light intensity even under a low power consumption (a small heat generation amount).
Structures described in Japanese Laid-Open Patent Publication Nos. 2007-026989 and 10-289602 have been proposed in view of preventing the illuminance reduction of the car light.
The structure described in Japanese Laid-Open Patent Publication No. 2007-026989 is obtained by attaching a heat generator containing a transparent insulating sheet and a conductive pattern printed thereon to a formed lens using an in-mold method. Specifically, the conductive pattern of the heat generator is composed of a composition containing a noble metal powder and a solvent-soluble thermoplastic resin.
The structure described in Japanese Laid-Open Patent Publication No. 10-289602 is obtained by attaching a heat generator to a lens portion in the car lamp. The lens portion is heated by applying an electric power to the heat generator under a predetermined condition. The document describes that the heat generator contains a transparent conductive film of ITO (Indium Tin Oxide), etc.